38 



agreement on this point, however; but, as most authorities class all 

 nitrogenous compounds with the albuminoids, we shall do likewise. 

 Fibrin in animals, gluten in grain, casein in milk, and legumin in 

 peas belong to this group, and serve, if in proportion, as nitrogenous 

 food. That is the great principle to bear in mind. 



2. The next class consists of fats and oih (often called hydrocarbons), 

 and is specially rich in carbon. A certain portion of fat is necessary for 

 the healthy body itself; so necessary that unless sufficient be supplied 

 a certain portion of albuminoids will be decomposed by the system in 

 order to form fat. Hence fat in due proportion is absolutely necessary in 

 order to prevent such a wasteful use of albuminoids. Besides this, fats 

 and oils find their chief work in supplying fuel for heat and energy. 



3. The next class, known as carbo-hydrates, consists of carbon (in less 

 proportion than in fat and oils), with hydrogen and oxygen in the propor- 

 tions of water. Starch, sugar > and gum are the leading compounds of 

 this class in the vegetable world. This group also supplies fuel for heat 

 and .energy, and has plainly more or less in common with the fats and 

 oils class. It differs, however, in this respect : it is not directly repre- 

 sented, as the fats group is, in the animal body itself. Carbo-hydrates 

 are, however, capable of being decomposed into fat. Thus they save 

 waste of albuminoid foods; in other words, a due proportion of the 

 carbon groups, as well as of albuminoids, is necessary even for the increase 

 or the formation of lean meat or muscle. 



4. One component of vegetable foods especially requires separate men- 

 tion. Cellulose, the material of which tough cell-walls and woody fibre 

 is composed, is of nearly the same chemical composition as starch. 

 (Paper and cotton-wool are examples of cellulose.) These and kindred 

 materials exist in a much more indigestible form, so much so that in the 

 case of some animals it is completely indigestible. Hence for our pur- 

 pose we take the harder of such constituents into a separate class, and 

 term it " husk " or " fibre. " A certain portion of these may be useful 

 as a mechanical stimulus to the intestines, but, except in the case of 

 ruminant animals or birds, there is no portion digested, and consequently 

 they are of little value as food. 



5. The last class is that of salts and minerals. Phosphorus and lime 

 are needed for the bones, sulphur for the feathers chiefly (the muscles 

 require a small quantity also), salt for the whole range of the digestive 

 processes, and alkaline salts to alkalinate the blood, &c. 



In addition to the five classes above mentioned, there is in all foods 

 a very variable amount of hydrogen and oxygen in the proportions that 

 form water, and may be classed as such, though the water as in the 

 case of apparently dry wheat or flour assumes in some way a solid 

 form and may not be water in reality. 



It is on the basis of these classes that food is analysed; and the 

 problem to be solved in feeding or in a dietary is of the simplest kind 

 from a theoretical point of view. It is to obtain a proper proportion 

 between the albuminoids, or nitrogenous compounds, and the heat-pro- 

 ducing groups of fats and carbo-hydrates. A dietary or food so arranged 

 is called a properly "balanced " ration; and if we give such a dietary 

 in proper quantity and in digestible forms the animal or bird will be 

 properly fed. The actual proportion in any food or any dietary is called 

 its " nutritive ratio." Thus, a mixture of meals whose nutritive ratio 

 is 1 : 6 means that the albuminoids therein are as one part (by weight) 

 to six parts of fats and carbo-hydrates. But in calculating this ratio one 

 important modification must always be made : " Fats " are much more 

 fattening than starch or other carbo-hydrates, and are more efficient 

 generally, because, as already noted, they are richer in carbon. In 

 adding up the two groups, therefore, we must multiply the figure for 



